This application claims the priority of Korean Patent Application No. 2003-4108, filed on Jan. 21, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method and apparatus for determining a zeta potential generated between a channel wall and a solution.
2. Description of the Related Art
When a glass microchannel or capillary is filled with an electrolyte solution, an electric double layer is formed at an interface between a liquid phase and a solid phase. If an electric field is applied tangentially to the electric double layer, an electric body force is exerted on the excess counter ions in the electric double layer, and thus an electroosmotic flow is generated. The electroosmotic flow is used as an important driving force in miniaturized analysis chips such as Lab-On-a-Chip (LOC). For the case where there is no pressure gradient between both ends of a channel, the Debye-Huckel theory is applied, and the electric double layer is much less than the characteristic length scale of the channel, the rate of the electroosmotic flow (u) is represented by Helmholtz-Smoluchowski equation as Equation 1:u=−(∈ζE)/μ,  Equation 1;
where ∈ is the dielectric constant of an electrolyte solution, ζ is the zeta potential at the electrolyte solution and the channel, μ is the viscosity of the electrolyte solution, and E is the electric field. The dielectric constant and viscosity of the electrolyte solution, which are physical property values of the electrolyte solution, are given as constant values with respect to the electric field. In this regard, provided that the zeta potential is determined, the rate of the electroosmotic flow in a channel can be obtained from a linear relationship between the rate of the electroosmotic flow and the external electric field. The rate of the electroosmotic flow serves as the most basic data for fluid control such as fluid separation and migration on the LOC.
Conventionally, the zeta potential of protein particles or particles dispersed in the dispersion system is mainly determined by measuring the mobility of particles. However, in order for the electroosmotic flow to be widely used as a driving force in miniaturized analysis chips, determination of the zeta potential generated between an electrolyte solution and a channel wall is required, rather than the determination of the zeta potential of particles. Methods of determining the zeta potential at a solid-solution interface are known in the art.
For example, U.S. Pat. No. 6,051,124 discloses a method of determining a zeta potential using a reflected laser beam. However, there is no mention to a solid.
Also, a particle tracking method is widely used in conventional fluid flow experiments. As a result of observation of tracer particles seeded in a fluid for a predetermined time, a straight particle trajectory is visualized. Since the displacement of the tracer particles for a predetermined time is given, the flow rate can be calculated. Therefore, a zeta potential can be determined by using Equation 1. However, tracer particles must be seeded to some degree in a zeta potential determining channel to ensure the zeta potential determination. Also, in a case where the tracer particles are electrically charged, an electrophoretic mobility due to the charged particles must be considered. In addition, since the wall surface of the previously used channel may be contaminated by the tracer particles, it is difficult to reuse the channel for additional experiments.
A zeta potential can also be determined by a current monitoring method based on the following principle [Anal. Chem. 1988, 60, 1837-1838]. When a capillary channel is filled with electrolytes with different concentrations and a voltage is applied to both ends of the channel, an electric current decreases or increases due to a change of the electrolyte concentration with time. When the distance between both ends of the channel is given and the time elapsed until there is no current change is measured, the rate of the electroosmotic flow can be calculated. Therefore, the zeta potential can be determined according to Equation 1. Due to simple experiment equipments, this method has widely been used for determining the zeta potential.
Determination of a zeta potential by a stream potential method is based on the following principle [Journal of Colloid and Interface Science 226, 328-339, 2000]. When a pressure gradient is induced at both ends of an electroosmotic determining channel, ions of an electric double layer on the wall of the channel are displaced, thereby causing an electric potential difference between both ends of the channel. When a steady-state is reached, a constant electric potential is maintained. This electric potential difference is called streaming potential. In order to determine the zeta potential using the streaming potential, data such as electroconductivity and pressure difference are required. In particular, regression analysis using multiple data set obtained by varying the length of the channel is required. For this reason, such a streaming potential method is relatively accurate, unlike the particle tracking method and current monitoring method. However, as mentioned above, because multiple data set for the regression analysis must be determined by previous experiments, the streaming potential method is not suitable for rapid measurement. Also, more experiment equipments to be attached to both ends of the channel are required, when compared to the above-described two methods.